Regular physical activity (exercise) can reduce pain in people with chronic musculoskeletal pain; whereas, unaccustomed exercise can exacerbate pain. This apparent dichotomy in pain response to physical activity is poorly understood, making exercise prescription for individuals with pain challenging. In sedentary mice, a single bout of exercise enhances the nociceptive response to subthreshold muscle stimuli (pH 5.0 saline); this enhancement is prevented by 5 days of voluntary running wheel activity. Further, in mice the development of chronic muscle pain, induced by repeated intramuscular acid (pH 4.0) injections, is prevented by 8 weeks of voluntary running wheel activity. Regular exercise is believed to activate central inhibitory pathways that produce an opioid-mediated analgesia; the rostral ventromedial medulla (RVM) is a key central nucleus in opioid-induced analgesia. However, little data is available to support a role for central opioid mechanisms in exercise-induced analgesia, particularly in conditions of chronic pain. Our preliminary data show that the analgesic effect of 8 weeks of running wheel activity (i.e. regular exercise) is reversed by systemic blockade of opioid receptors, establishing that opioids are important in regular exercise-induced analgesia. Our preliminary data show that there is increased p-NR1 (NMDA receptor) in the RVM in sedentary animals after a single-bout of exercise or induction chronic muscle pain. These increases in p-NR1 in the RVM, however, are prevented by regular exercise, suggesting that p-NR1 is modulated by mechanisms activated by exercise. These data led to our central hypothesis that regular exercise enhances activation of central inhibitory pathways that utilize endogenous opioids to modulate p-NR1 in the RVM. We will address our central aim through the following specific aims. Aim 1 will determine if regular physical activity (running wheel exercise) prevents the development of chronic muscle pain, and if such an effect is associated with motor and autonomic responses that might occur in response to exercise training and activation of the RVM. Aim 2 will determine if regular physical activity prevents the development of hyperalgesia by activation of opioid receptors. We will test this by pharmacological and genetic manipulation of opioid receptors. Aim 3 will explore the neural circuitry involved in the enhanced nociception to unaccustomed physical activity and the analgesia produced by regular physical activity. We will establish if NMDA receptors are located on and modulate pain facilitatory ON cells through m- opioid receptors (MOR), and if these cells project to the spinal cord. These studies will be the first to evaluate the effects of regula exercise on hyperalgesia and the underlying mechanisms that mediate these effects. Understanding these interactions will give us a better understanding of the underlying neurobiology to improve the overall management of people with chronic musculoskeletal pain, and prevention of development of chronic pain.